The present invention relates to a process for the gas phase polymerization of olefins in a fluidized bed reactor and in particular to the introduction of a deactivating agent into the reactor without terminating the polymerization reaction.
It is known to polymerise one or more alpha-olefins, such as ethylene or propylene, in the gas-phase in a fluidized bed reactor, in the presence of a catalyst based on a transition metal belonging to the groups IV, V or VI of the Periodic Table of the Elements; in particular in the presence of a catalyst of the Ziegler-Natta type, chromium oxide type or a metallocene catalyst. Catalyst particles, together with growing and formed polymer particles are kept in the fluidized and/or agitated state in a gaseous reaction mixture containing the alpha-olefin or alpha-olefins, which are introduced continuously into the reactor. The catalyst is introduced continuously or intermittently into the reactor while the polymer constituting the fluidized bed is withdrawn from the reactor, also continuously or intermittently. The heat of the polymerization reaction is essentially removed by the gaseous reaction mixture, which may be passed through a heat transfer means before being recycled into the reactor.
Fouling of the reactor wall is a well-known phenomenon in gas phase polymerization processes. Hot spots and hence fouling are likely to occur in the vicinity of, especially at or close to the reactor walls. Where a polymerization reactor is equipped with a fluidisation grid the hot spots/fouling typically occur in the region 0.25 D to 0.75 D above the grid (where D is the diameter of the reactor). Hot spots/fouling may also occur in the disengagement zone. Such hot spots/fouling are usually detected by the use of thermocouples or temperature indicators either attached to the reactor wall or inserted a small distance into the reactor itself. Such detection means may be associated with means for recording temperature variations and possibly visual and/or audible alarms. The explanations for the occurrence of fouling are many and varied. Operating conditions, even small variations in the reaction conditions have been considered important to the occurrence of fouling. For example, poor or loss of fluidizing gas flow can cause catalyst particles, formed polymer particles and growing polymer particles to be insufficiently cooled by the gaseous reaction mixture passing through the reactor. This insufficient heat removal gives rise to hot spots, which can lead to the melting of the catalyst/polymer particles resulting in the formation of agglomerates of molten catalyst/polymer which then may melt on and cause fouling of the reactor walls. Other reasons given for the occurrence of fouling include the type of catalyst used and the presence of static electricity. Whatever the explanation of its occurrence the presence of fouling very often induces fluidization pertubations which can lead to irreversible process problems. For example, when the agglomerates become heavy, they can come off the wall and block the fluidization grid and/or the polymer withdrawal system. Severe hot spot and therefore fouling problems may necessitate a shut-down of the reactor. One way in which the polymerization process can be terminated quickly to avoid irreversible process problems and/or a reactor shut down is to introduce a deactivating agent whose purpose is to kill or terminate the fluidized bed polymerization.
European Patent EP-B-0 471 479 discloses a process for completely stopping a gas-phase alpha-olefin polymerization reaction carried out with the aid of a chromium oxide-based catalyst by introducing a deactivating agent such as oxygen, ammonia or carbon monoxide into the polymerization reactor.
U.S. Pat. No. 4,306,044 discloses a system for introducing carbon dioxide into a gas-phase olefin polymerization process to at least reduce the rate of the polymerization reaction.
Thus there remains the need to reduce or even eliminate hot spots/fouling in gas-phase olefin polymerization processes without terminating the polymerization reaction.
It has now been unexpectedly found that hot spots and hence fouling can be reduced or even eliminated by using the process according to the present invention without the need to stop production or shut down the reactor.
Thus, according to the present invention there is provided a process for the gas phase polymerization of one or more alpha-olefins in the presence of a polymerization catalyst system under reactive conditions in a fluidized bed reactor which process comprises introducing into the reactor a deactivating agent characterized in that the deactivating agent is introduced close to the reactor wall and that said introduction does not terminate the polymerization reaction.